| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.331.102.2011 PetrologyThe methodology used during Expedition 331 for visual description of sedimentary and igneous lithological features is outlined in “Lithostratigraphy” and will not be repeated here. However, many of the sediments and rocks recovered during Expedition 331 have undergone hydrothermal alteration and/or sulfide mineralization. The procedures used for visually describing these intervals are detailed in the following sections. In addition, this section describes the more advanced petrographic and analytical techniques utilized by the shipboard party to characterize recovered cores, namely X-ray fluorescence (XRF) geochemistry, XRD mineralogy, polished thin section description, and SEM analysis. Visual description of hydrothermally altered materialDuring Expedition 331, all rocks logged were graded according to whether they are fresh (<2% by volume alteration products) or have slight (2% to <10%), moderate (10% to <40%), high (40% to <80%), very high (80% to <95%), or complete (95%–100%) alteration. Additionally, through incorporation of VCDs with the results of XRD mineralogical analysis, the style of alteration for each altered interval was determined. A list of alteration styles and their features is given in Table T2. Alteration intensity and style were tabulated in Microsoft Excel by core, section, and interval for all holes drilled and included on the section-scale VCDs using Strater software. Symbols used for alteration logging are given in Figure F4. Visual description of sulfidesSeafloor and subseafloor sulfide mineralization can occur as massive sulfides; semimassive sulfides; sulfide minerals in stockwork, stringers, or veins; sulfide filling voids in rocks; sulfide-filled breccia; disseminated sulfides; and metalliferous (sulfide and oxide) sediments (Table T3). Sulfide mineralization styles and sulfide and sulfate mineral species were observed visually and recorded in section-scale templates using the shipboard database software J-CORES. A list of sulfide and sulfate minerals observed during Expedition 331 and their chemical formulas is given in Table T4. Visually estimated sulfide abundance was recorded in the written descriptions given for all lithological intervals in J-CORES and then tabulated in Microsoft Excel by core, section, and interval for all holes. Sulfide and sulfate mineral distribution and sulfide abundance were then included on the section-scale VCDs using Strater software. Symbols used for sulfide logging are given in Figure F4. X-ray diffractionWe completed XRD analyses of bulk powders using a PANalytical CubiX PRO (PW3800) diffractometer. Samples were routinely taken from each 1.5 m core section during hydraulic piston coring system (HPCS) and extended shoe coring system (ESCS) drilling and from every second 0.75 m core section for Baker Hughes INTEQ (BHI) cores. Additional samples were selected at irregular intervals where needed to represent the range of lithologies encountered during drilling and provide a more quantitative context for visual core description. Our principal goal was to determine the minerals present in the very fine grained material that made up the bulk of recovery during Expedition 331. Data processing was carried out using X’Pert HighScore Version 2.1 (2.10), which is produced by PANalytical B.V. (Almelo, Netherlands). X’Pert HighScore matched peaks from the unknown sample to those of known crystal structures from the Inorganic Crystal Structure Database, which contains thousands of XRD patterns from known synthetic and naturally formed phases. The majority of these reference patterns were gathered from single crystal analyses, which does not allow for the effects of preferred orientation or stacking disorders. These effects can be particularly significant for clay minerals and phyllosilicates. The program therefore provided a guide to potential phases, which were then interpreted by the operator. Interpreted mineralogies were tabulated, in approximate order of abundance, for each sample analyzed and were included in the site reports. Peak heights may be strongly influenced by factors other than abundance, so no quantitative measurement is implied. In particular, abundances of clay and phyllosilicate minerals are not quantitatively estimated and are likely to be underestimated. Samples were freeze-dried, crushed with a ball mill, and mounted as random bulk powders. Instrument settings were as follows:
X-ray fluorescenceThe lack of technical support for preparation of glass beads and the small size of the core description team for Expedition 331 meant that only four samples were prepared for XRF analysis. Major elements were measured using the fused glass bead method and are presented as weight percent oxide proportions (SiO2, TiO2, Al2O3, Fe2O3, MnO, CaO, MgO, Na2O, K2O, and P2O5). An aliquot of 0.9 g of ignited sample powder was fused with 4.5 g of SmeltA12 flux for 7 min at 1150°C to create glass beads. Loss on ignition was measured using weight changes on heating at 1000°C for 3 h. Analyses were performed on the wavelength dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA. Analytical details and measuring conditions for each component are given in Table T5. National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan) rock standards were used as reference materials for quantitative analysis. Table T6 lists the results and standard deviations for selected standard samples. A calibration curve was created with matrix corrections provided by the operating software, using the average content of each component. Processed data were uploaded into the J-CORES database. Polished thin section descriptionThe lack of technical support for thin section preparation and the small size of the core description team meant that only six polished thin sections of competent lithologies could be prepared on board the Chikyu for microscopic studies of mineralogy, petrology, internal structures, and fabric. A thin section was prepared as a 30 μm (0.03 mm) thick slice of core or cuttings sample. The standard size of billets for thin section preparation was 2 cm × 3 cm × 0.8 cm. Four of the six samples sectioned were strongly altered and porous, so they were dried first in the oven at 105°C and then impregnated under vacuum (Epovac) with epoxy (Epofix) prior to mounting. Polished thin sections were observed in transmitted and reflected light using an Axioskop 40A polarizing microscope (Carl Zeiss) equipped with a Nikon DS-Fi1 digital camera. Scanning electron microscopyAqueous sediment samples were fixed using 2% glutaraldehyde, dehydrated using acetone, and embedded in Epon resin. Friable hard rock samples were embedded directly in Epon to stabilize the material. Nonfriable rock was prepared as conventional polished thin sections. Samples were subsequently ground, polished, and platinum coated using a JEOL JFC-1800 nano fine coater. All samples were viewed on a JEOL 5770 SEM equipped with both backscattering and X-ray spectroscopic (energy-dispersive spectrometry [EDS]) detectors operating at an accelerating voltage of 15 kV. |
|||
